Thermoset injection molding systems such as reaction injection molding (RIM), structural reaction injection molding (SRIM), resin transfer molding (RTM), etc., are used widely throughout industry to form parts of varying shapes and complexity. In thermoset injection molding, a thermosetting material such as a polyester or epoxy resin is injected into a mold having an interior cavity that defines the shape of the formed part. Sufficient resin is injected into the mold to allow the resin to completely fill the interior cavity of the mold and to force air within the mold out. It is important that the injection mold and the injection operation be carefully designed, monitored and controlled to ensure that all the air is driven out of the mold; otherwise, air may become trapped within the mold cavity. Air trapped within the mold causes areas of porosity and voids in the molded part. Often, these areas of porosity affect the structural integrity or aesthetics of the part, causing the part to be rejected as commercially unacceptable.
The strength, stiffness, and other structural characteristics of thermoset injection molded parts are determined by the structural design of the part, the type of resin used, and the type of internal reinforcement used. Modern resin systems and reinforcements are used to produce injection molded parts in a wide variety of different industries, including the automobile, truck and airline industries. Often, injection molded parts used in the automobile and airline industries are highly reinforced, either by structural reinforcements such as webs or beads, or through the use of fibrous reinforcements integrally molded into the parts. As structural reinforcements or internal fibrous reinforcements are added to an injection molded part, the part becomes more difficult to fabricate and is more prone to the formation of voids or areas of porosity.
In RIM and RTM systems, a multicomponent thermosetting polymer resin system such as a polyurethane resin material system is generally used. A typical polyurethane resin system consists of two components, one being a blend of polyols or blends of polyols containing catalysts and surfactants and a second component being an isocyanate or isocyanate blend. The multicomponent polyurethane resin system is generally mixed upon injection into the mold via a high-pressure impingement mixing system. The high pressure impingement mixing system is generally placed immediately adjacent the injection port on the mold because upon mixing, the polyurethane resin begins to chemically react. As the resin reacts, it expands dramatically, often increasing in volume by up to 30 times in an unrestricted environment.
In RIM and RTM systems, the mold is generally maintained at room temperature or a slightly elevated temperature, and the resin is injected at a relatively low pressure. A typical mold temperature used in RIM systems is 30.degree. to 90.degree. C. The expansion of the polyurethane resin during the chemical reaction provides the majority of the force necessary to fill the interior cavity of the mold. The magnitude of the pressure produced in the mold by the expanding resin is dependent upon the resin system used, the type and density of reinforcements used, and the amount of blowing agent used. Blowing agents determine the amount of foaming, i.e., expansion, of a polyurethane resin system during the chemical reaction process.
In some RIM systems, the expanding polyurethane resin system placing approximately a 10 to 50 psi force against the interior mold surfaces. This expansion force is generally sufficient to ensure that the expanding resin system fills the interior cavity of the mold.
The time to fabricate a part using RIM systems differs depending upon the resin used, the injection rate, the part size and the part geometry. Once injected, a typical resin system used in RIM will cure within 30 seconds to several minutes. The completed part may then be immediately removed from the mold.
The density of a completed RIM part is adjusted by altering the amount of resin material used in conjunction with the amount of blowing agent added to the resin system as it is injected into the mold. As an example, polyurethane resin systems can be reaction injection molded to produce pans having a density of anywhere from 2 to over 65 lbs. per cubic foot. Polyurethane resin systems using little or no blowing agent, thus producing a dense part of 50 lbs. per cubic inch or more, are commonly referred to as "high density parts," while resin systems containing more blowing agents and thus producing parts having a lower density of approximately 45 lbs. per cubic foot or less are commonly referred to as "low density parts."
The application of RIM systems was generally limited to forming parts having fairly low structural load requirements until the creation of "structural reaction injection molding" (SRIM). In SKIM systems, a structural reinforcement is placed within the mold cavity prior to injecting the resin system. Often, the structural reinforcement used is a woven or nonwoven mat, cloth, veil or roving. Such reinforcements can be formed of fiberglass, kevlar, graphite, or other reinforcing fibers. As the resin system is injected, it flows around and impregnates the fibrous reinforcement, forcing air out of the reinforcement and mold to produce a composite structural part. SRIM results in pans having an increased stiffness, toughness and strength relative to standard RIM pans.
All thermosetting injection molding systems including RIM and SRIM and RTM systems can be used to make pans of differing complexity. Often thermoset injection molded pans include ribs, stiffeners, beads, or other complex structural details. One recurring problem with all injection molding methods when used to form complex pans is areas of porosity or voids. When the resin fails to force all the air out of the mold and any reinforcements contained within the mold, air pockets are trapped within the part. These air pockets create voids or areas of porosity on the surface of or within the formed part. Voids and areas of porosity not only detract from the parts' aesthetics, but also reduce the structural integrity of the part depending upon the location and size of the void.
Voids or areas of porosity are especially prevalent in SRIM pans due to the increased resin flow restriction caused by the fibrous reinforcements within the part. The reinforcements used in SRIM are generally fairly dense and of a woven or continuous strand glass mat nature, thus they impede the flow of the resin when it contacts and flows through the reinforcements during molding. The greater the percentage volume of reinforcements used, the harder it is for the resin to flow through the reinforcements, thus the greater the occurrence of voids or areas of porosity.
Depending upon the location and type of fibrous reinforcements, the resin can be channeled around the reinforcement as the resin follows a path of least resistance, thus increasing the occurrence of voids. In addition to the fibrous reinforcements, structural shapes such as ribs, channels, and beads often create paths of reduced resistance through which the resin flows, often resulting in air being trapped within the molded part. Because of the increased occurrence of voids in complex parts having fibrous reinforcements, ribs, channels and beads, complex shapes have not generally been formed using SRIM.
To reduce the formation of voids in all methods of thermoset injection molding, vents are strategically placed to exhaust air from the mold as it fills with resin. In simple symmetrical flat parts, the mold is generally filled with resin through an injection port in the center of the part. The resin then flows concentrically outwardly, forcing the air out vents located at the parting line in between the upper and lower portions of the mold. In simple flat parts, the resin flow pattern can be easily predicted and, thus, the vents can be located to ensure that no air is trapped in the part. In more complex parts, the resin is often injected into the mold at a location other than the geometric center. Off-center injection ports are generally used in parts that are not symmetrical or have shapes such as ribs, channels, or beads that tend to cause an uneven flow of resin. The more complex the part's shape, the more complex the resin flow pattern. Adding internal fibrous reinforcements such as those used in SRIM parts makes the resin flow pattern even more uneven.
If the resin flow pattern is known, the vents may be placed at strategic locations within the mold in an attempt to allow air to escape the mold as it is filled with resin, thus reducing the formation of voids. However, in complex parts, especially those containing internal fibrous reinforcements, it is impossible to accurately predict the resin flow pattern. In addition, the resin flow pattern may differ during successive molding cycles even if the interior shape of the mold and the molding parameters such as temperature, flow rate, etc., remain constant. As the resin flow pattern changes, so does the location of voids within the port. Thus, it is difficult if not impossible to locate the vents at locations to consistently eliminate all voids and areas of porosity in a complex part.
One way to reduce the number of voids in an injection molded part is to vent large amounts of resin through the vents. As the resin flows out of the vents, it carries some of the air trapped within the mold out. However, venting resin out the vents is extremely wasteful, adds significantly to the cost of the molded part, and generally does not eliminate all voids in a complex part.
An improved method of venting air from the mold during forming would allow more complex parts to be fabricated having a reduced number of voids. An improved method of venting air within the mold would also allow higher quality parts to be formed without excessive venting and waste of the resin, thus decreasing part costs. One goal of the present invention is to reduce some of the disadvantages of prior art thermosetting injection molding methods discussed above.